1. Field of the Invention
This invention relates to sheet-fed printing presses and, in particular, this invention relates to an apparatus and a method to cool printing press cylinders, such as impression cylinders and transfer cylinders, and substrates being printed thereon.
2. Background of the Invention
Sheet-fed printing presses use such processes as lithographic, flexographic, and gravure printing to transfer images to a substrate such as plastic or paper. The sheet substrate is usually conveyed through the printing press by a series of rotating transfer and impression cylinders (collectively transfer elements). The image is usually transferred by depositing liquid or paste ink onto the substrate from a blanket cylinder, raised image plate, or gravure cylinder to the impression cylinder while the substrate is positioned between surfaces of the blanket cylinder and the impression cylinder. After being printed, the image of liquid or paste ink (or coating) may be dried or cured by radiant energy. The radiant energy is usually directed at the newly printed substrate while the substrate is positioned over the surface of the impression or transfer cylinder. Thus, both the substrate and printing cylinders absorb heat. Some of the heat cures the ink and raises the temperature of the substrate, which is conveyed away from the radiation site. Some of the heat absorbed by the printing cylinders is dissipated by radiation. However some of the heat absorbed by the printing press cylinders raises the temperatures of these elements and is then transferred to substrates. The heated substrates respond to this heat transfer by distortion (e.g., expansion and contraction).
The typical finished printed image is actually a mosaic of several component images deposited sequentially by multiple printing units. Thus, each printing unit adds to the final image. For example, an image with a blue background on white paper and with red lettering requires exact spaces left blank for the red lettering. Each component image must be transferred to an exact position on the substrate. For example, between about 133 and 600 lines per inch are from common registration criteria. If the substrate expands or contracts during the printing process, a phenomenon known as thermal distortion occurs and component images are not transferred to exact positions. When this occurs the finished product is often blurred or distorted. The above-mentioned substrate distortion, therefore, is a result of substrate heat absorption via the curing process and of printing press cylinders operated at temperatures above a critical temperature range.
Thus far, cylinders in printing units have been maintained within desired temperature ranges by utilizing cool air ducted into the press from an external air conditioning unit. Maintaining the cylinder temperatures by this method is expensive, inefficient, and often ineffective. These prior art temperature control systems require large amounts of energy to cool air from ambient temperatures, to often as low as 35 degrees F. Moreover and due to space constraints within the printing press, the ducts often cannot be installed so as to direct cool air to all printing surfaces, which are being operated at elevated temperatures. Because these ducted systems often fail to cool all printing cylinders, the final product is frequently flawed by thermal distortion.
There is then a need for an apparatus and a method for quickly and efficiently maintaining printing cylinders within temperature ranges which will not cause substrate distortion. There is a particular need for an apparatus and a method to economically and effectively maintain printing press cylinders at temperatures (e.g., below 100 degrees F) to insure that an optimum quality printed product is achieved.
This invention substantially meets the aforementioned needs of the industry by providing a printing system, the printing system including a blanket cylinder, transfer elements such as impression (printing) cylinders and~transfer cylinders, and a unit for directing an airflow at the impression. The present airflow temperature range may be a temperature, which will effectively prevent the substrate from being thermally distorted beyond a specified limit. The effective airflow temperature may also be below a maximum acceptable temperature of the transfer element. Under certain circumstances, the present effective airflow temperature may be between slightly above 32 degrees F. (e.g., 34 degrees F.) and 120 degrees F.; between 40 degrees F. and 75 degrees F., or between 50 degrees F. and 55 degrees F.
The cooling unit may include at least one fan and at least one cooling coil, the cooling coil positioned to cool air directed by the fan at the transfer element. The cooling coil may convey a coolant such as a chilled liquid or a compressible gas. The cooling unit may further include a temperature sensor in electrical communication with the fan for measuring a transfer element temperature. The sensor may be an infrared, noncontact sensor. The present invention may further include a control unit in electrical communication with the sensor, the fan, and a pump or valve. The control unit activates the fan and the pump or valve when the transfer element is heated to a designated temperature, the designated temperature detected by the sensor. The pump or valve then circulates coolant through the cooling coil.
There is further provided a process for maintaining a transfer element within a designated temperature range, the process including providing a cooling system and directing an airflow at the transfer element. The cooling system may include at least one fan and a cooling coil. The airflow may be cooled by the cooling coil and generated by the fan. The cooling coil may convey a refrigerant such as chilled water or a compressible gas.
There is yet further provided a process of installing a cooling unit for a printing cylinder, the process including providing the cooling unit and positioning the cooling unit so as to direct an airflow at a transfer element. The cooling unit may include at least one fan, a cooling coil, and a pump or valve. The fan may generate an airflow, which is cooled by the cooling coil and directed at the transfer element. The pump may circulate a cooling fluid through the coiling coil.
An object of this invention is to provide an apparatus to maintain a transfer element within a temperature range effective to ensure that substrate distortion is within allowable limits.
Another object of this invention is to retrofit a printing assembly with an apparatus to maintain transfer elements therein within an effective or desired temperature range.
It is an advantage of this invention that transfer element temperatures can be maintained within desired temperature ranges with less energy than was previously possible.
It is another advantage of this invention that each transfer element can be maintained within a desired temperature range notwithstanding space limitations within the printing press unit.
It is yet another advantage of this invention that each transfer element can be maintained within a desired temperature range without the expense and effort previously required to install air ducts.
Additional objects, advantages, and features of various embodiments of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of various embodiments of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.